Update to 0.55.6

Changelog:
This is a bug-fix release with one important fix. There have been
reports about infrequent paginator crashes when running the Hugo
server since 0.55.0. The reason have been narrowed down to that of
parallel rebuilds. This isn't a new thing, but the changes in 0.55.0
made it extra important to serialize the page initialization. This
release fixes that by protecting the Build method with a lock when
running in server mode. 95ce2a40 @bep #5885#5968

(ryoon)